1. Field of the Invention
The invention relates to a method and a device for generating steam with a conventional steam turbine cycle provided with a steam generator, in which a first relaxation stage in a high-pressure turbine is followed by an intermediate superheating of the steam prior to a second relaxation stage in a medium-pressure turbine, whereby the steam turbine cycle optionally can also be combined with a gas turbine cycle provided with at least one gas turbine, in which gas turbine cycle the gas turbine is followed by a waste heat steam generation plant supplied with water from the steam turbine cycle, and the steam-side output of the waste heat steam generation plant and the steam-side output of the steam generator join upstream from the high-pressure turbine of the steam turbine cycle to flow into a common live steam line.
2. Brief Description of the Related Art
Conventional steam power plants essentially consist of a steam generator, which is powered mostly with coal or oil, but increasingly also with gas, and of several steam segment turbines (high-pressure, medium-pressure, low-pressure steam turbines), as well as a generator for converting the steam energy into electrical energy. To increase the efficiency, it is common practice to perform an intermediate superheating of the steam relaxed in the high-pressure turbine before it is fed to the medium-pressure turbine.
The temperatures of the superheated and intermediately superheated steam vary in this state of the art depending on the boiler load. For a low boiler load, the temperature of the superheated steam is higher than that of the intermediately superheated steam; for a high boiler load, the temperature of the intermediately superheated steam is higher than the temperature of the superheated steam. The intermediate superheater is designed for part of the live steam/HP superheater steam throughput, since in a conventional steam power plant, the steam is bled for the regenerative preheating, and the throughput in the steam turbine is continuously reduced up to the steam turbine discharge/condenser. The throughput of the superheater is therefore much higher for a conventional steam power plant than the mass stream through the intermediate superheater. For this reason a reduction of the intermediate superheater temperature must be achieved in the case of higher boiler loads.
Since the heat exchange surfaces of superheater and intermediate superheaters are fixed in a particular case, the steam temperature must be regulated, i.e. it must be maintained constant within specific limits (maximum temperature depends on material; minimum temperature depends on output to be achieved). This may be accomplished, for example, by changing the fuel system/rotating the burners, by steam cooling based on water injection, by recycling of flue gas or by bypassing heat exchange surfaces (guide baffle regulation).
However, these known solutions for regulating the steam temperature have several disadvantages. On the one hand, they have only a limited effect; on the other hand, they require the installation of additional hardware, thus increasing cost. In the case of steam cooling by water injection between or after the superheater or intermediate superheater sections, the performance will also be reduced. The devices, such as guide baffles which move under extreme conditions (high temperatures, corrosion), also have a disadvantageous effect.
DE 195 42 917 A1 and R. Bachmann, M. Fetescu, and H. Nielsen: More than 60% Efficiency by Combining Advanced Gas Turbines and Conventional Steam Power Plants, Power Gen '95 Americas, Anaheim, Calif., U.S.A., Dec. 5-7, 1995, describe, for example, combined power plants in which a steam cycle, like the one described above, is combined with intermediate superheating with a gas turbine cycle, whereby the gas turbine is followed by a waste heat boiler which generates additional live steam from part of the feed water. This additional live steam from the waste heat boiler has the result that the live steam mass stream discharged from the main boiler must be smaller than the live steam discharge from the boiler of a conventional steam power plant. Since the preheating of the condensate and feed water in the waste heat boiler additionally reduces the bleeding volume of the relaxed steam from the LP steam turbine, the steam throughput through the steam turbine is increased, so that the boiler load must be reduced. As a result, the cold intermediate superheater mass steam of a combined system is much greater than the live steam mass steam of the boiler, thus creating a disproportion between them.
In the known state of the art, the live steam generated in the main boiler and in the waste heat boiler is only intermediately superheated in the main boiler. Although this has a number of advantages, such as, e.g., enabling high flexibility in the operating mode while maintaining very high efficiency, this also has disadvantages. The superheater of the main boiler is operated at a partial load, and the intermediate superheater is operated at the higher base load. If the combined power plant operates without any modification, this has the result that the steam temperature is reduced at the outlet of the intermediate superheater, and the medium-pressure turbine output, and accordingly the efficiency of the power plant, are reduced.